1. Field of the Invention
The present invention relates to an electrophoretic display device that displays using the migration of charged particles.
2. Related Background Art
With the remarkable progress in digital technology, the amount of information a person can handle has been increasing rapidly. In connection with this, displays as an information output means are actively developed aiming at a display of high usability such as high definition, low power consumption, light weight and thin body. In recent years, large expectations are laid on the “easy to read” reflective display having the same display quality as the printed matters, which is a technology indispensable for the next-generation products such as electronic paper and electronic books. A candidate of such a display is an electrophoretic display device proposed by U.S. Pat. No. 3,612,758 to Harold D. Lees et al.
This electrophoretic display device comprises a pair of substrates and a mixture of colored charged particles and an insulating liquid containing a coloring agent held between the substrates, to form an image with the color contrast between the coloring charged particles and the colored insulating liquid.
Japanese Unexamined Patent Publication No. H11-202804 proposes an electrophoretic display device which does not require coloration of the insulating liquid and forms an image with the color contrast between the colored electrophoretic particles distributed in a transparent fluid and a colored layer or reflective layer located on a substrate. This display uses the difference of appearance when the particles are in a position to cover the colored layer and when they are in a position to reveal the colored layer. In order to move the charged particles laterally, one electrode is located on one substrate, and the other electrode is located under the colored layer. In many cases, an electrode surface under the colored layer also serves as a light reflector to reflect the light that enters from above the substrate to the upward direction.
Such an electrophoretic display device uses a transparent liquid without mixing a coloring agent such as a dyestuff into the insulating liquid. This has benefits such as elongation of display life and prevention of contrast deterioration.
For an easy-to-read reflective display device having a display quality equivalent to printed matters, brighter display quality is required. Conventional reflective type electrophoretic display devices have a paper-like readablility, but become invisible in a pitch-black place with no outside light. In particular, if the surrounding environment changes from a bright environment to a very dark environment temporarily, for example, when a user moves from outdoor to indoor, the user cannot visually recognize the display in use, which is a disadvantage in practical use.
In order to solve such a problem, Japanese Unexamined Patent Publication No. 2002-040489 proposes to mix a fluorescent material into the insulating liquid. In addition, Japanese Unexamined Patent Publication No. 2002-287174 proposes to mix a fluorescent material in the charged particles.
However, when a fluorescent material or a luminous material is added to the charged particles or the insulating liquid, there arise many restrictions on process and material selection, in consideration of, for example, control of the content and distribution of the fluorescent material or luminous material in the charged particles, electrochemical stability with a charge controlling agent or a coloring agent, solubility to the insulating liquid and the like.
Then, the present invention intends to solve the problems mentioned above, and aims at providing an electrophoretic display device of reflective type, and which has relatively fewer restrictions on production process, and enables visual recognition of information even when there is almost no surrounding light, as well as the enhanced reflectance.